In the operation of molding presses for certain materials such as rubber, it is desirable to subject the press mold to repeated, alternate application and release of pressure in order to allow the escape of air and gases that may have become entrapped in the mold or to permit a rearrangement of the material in the mold. This repeated, alternate application and release of pressure is commonly referred to as "bumping", each "bump" being obtained by reducing the press clamp pressure for a short interval to allow the air and gases to blow their way out of the mold or by actually opening slightly the mold to create a slight gap through which the air and gases may escape. If the air and gases are not allowed to escape, surface blemishes on the molded part or other irregularities may occur and result in scrapping of the defective part.
Preferably, a plurality of bumps are performed in accordance with a prescribed sequence before final closure of the press and application of cure pressure in order to ensure expulsion of all entrapped air and gas and to progressively work the material into the mold cavity or cavities, this being particularly important where the mold cavity or cavities contain complex contours. Typically, the bumps are performed at progressively greater pressure during closing of the mold. However, the pressures at which the bumps are performed and the total number of bumps to be performed may vary from job to job depending, for example, on the viscosity of the particular rubber compound used and the contour of the mold cavity or cavities. It has been found that the proper bumping operation with some rubber compounds is highly dependent on the rubber viscosity in that it affects the degree to which the rubber fills the mold cavity or cavities at any given pressure.
Heretofore, such bumping of the press has been manually accomplished by the press operator merely releasing or dumping the hydraulic pressure to the press ram which in turn releases the clamp pressure acting upon the mold. After a short interval, the press operator would again apply hydraulic pressure to the press ram until a next bump is to be performed. As is often desired, the press operator would dump pressure at progressively greater pressures during closing of the mold, the material being further worked into the mold cavity by each progressively greater pressure. In order to accomplish this, the press operator would watch a pressure gauge to determine when to dump pressure, i.e., to effect a bump.
This procedure however has several drawbacks. In particular, such procedure is subject to human error and inconsistency. As the desired bumping operation may vary from one job to another, it is not uncommon for the press operator to follow an improper bumping sequence. Further inconsistencies occur as a result of shift changes and turn-over of personnel. Moreover, the reaction time of the press operator will not allow for precise repeatability. Accordingly, there is no assurance of repetition of the same procedure for each press cycle, and this all leads to high scrap rates.
Still another drawback of such procedure is that in a production environment the operator may have to prepare for his next run immediately upon closing the mold in the press. It is likely that in trying to perform next run preparations in addition to bumping the press, the operator will miss the proper pressure at which trapped air and gas should be released. Of course, the operator could concentrate solely on the bumping operation and upon its completion turn his attention to next run preparations, but this would cause lost production time. If he could be freed of the bumping task, he could immediately turn to next run preparations and/or perform another task or tasks thereby resulting in increased productivity.
Control apparatus for effecting automatic bumping of a press are known. However, such control apparatus either bump the press at specified time intervals after the press is closed, at specified degrees of closure or only at a single pressure for a selected period of time. In those control apparatus which bump the press at specified degrees of closure, it has been difficult to set the mechanical control elements thereof to obtain a specific relation between cavity pressure and the bump. Even if the proper pressure-distance relationship could be precisely set, any change in rubber viscosity would require recalibration of the distance oriented control elements. That is, as rubber viscosity changes, the amount of press travel to reach a specified pressure will also change.
In those control apparatus which bump the press at specified time intervals, there again is generally no specific relation between cavity pressure and the pump nor is there any compensation for viscosity changes. Because of rubber viscosity variances, the point in time at which a specific pressure will be reached will change. Moreover, the repeatability of such apparatus is disadvantageously affected, for example, by variations in the power supply to the timers employed therein as well as press hydraulic fluid viscosity changes due to the accumulation of heat therein which vary the speed of the press.
A control apparatus which bumps the press a number of times at the same pressure for a selected period of time is disclosed in U.S. Pat. No. 2,367,242. Such apparatus however does not provide for controlled sequential bumping of the press at different pressures.
Once the bumping operation is completed and the press is finally closed, typically full pressure is applied to the mold parts during heating and curing of the material therein. This however may result in a development of substantial residual stress in the molded part because the thermal expansion forces generated by the material as it heats up cannot overcome the press clamp pressure acting on the mold parts. With some materials, this may result in the formation of fractures in the rubber at the parting line of the mold. The formation of such fractures in this manner is commonly referred to as "back rinding".
For those materials that are subject to back rinding, it is desirable to cure same at low pressure. As a result, the mold parts can yield slightly to the thermal expansion forces of the material and thus prevent back rinding. However, relatively high pressure desirably must first be applied to the mold parts to obtain complete filling of the mold cavity or cavities and exhausting of excess material such as by extrusion at the mold part line. Accordingly, it would be desirable to bump the press at progressively greater pressures up to a high pressure and then set back the pressure to a desired low pressure for cure.